Hello,

In order to change Header Shop banner, navigate to Theme Settings > Shop > Header Banner and change http://prntscr.com/j8a6im

In order to fix the problem with Category icons, you need to open your SVG icons file with some editor like Sublime Text or default system text editor and add height and width attributes to your <svg> tag like it is shown on the screenshot https://gyazo.com/83fd36b0bb9628bb1aed2bd3d5d063c5
Then you will need to re-upload your icons in WordPress Dashboard.

Best Regars